WASHINGTON — The White House and Pentagon have been engaged in a series of talks throughout the day about potential military options against the Syrian regime in response to its alleged chemical attack on civilians earlier this week.

It was a dramatic turn of events for much of the military, which had no idea that the White House was considering its options when President Donald Trump first publicly mentioned a possible US response a day earlier.

The Pentagon is “in the process of presenting options,” a US official told BuzzFeed News. “There is a ton of traffic.”

Secretary of State Rex Tillerson speaking to the press said "we are considering an appropriate response for this chemical weapons attack." Tillerson emphasized the new administration stance, saying there was "no role for Assad" in the future of Syria, declaring that there would be a "serious matter, that requires a serious response."

“I think what happened in Syria is a disgrace to humanity, and he’s there, and I guess he’s running things, so I guess something should happen,” Trump told reporters aboard Air Force One, en route to Florida for a meeting with China's president. “What Assad did is terrible. What happened in Syria is truly one of the egregious crimes and it shouldn’t have happened. And it shouldn’t be allow to happen.”

“I don’t want to say what I’m going to be doing with respect to Syria,” he said, when asked about what the US response might look like.

At least 70 people were killed in the attack on Idlib, including at least 10 children, and hundreds injured. Many suffocated to death by the foam that formed around their noses and mouths.

The military options range from striking the Syrian air force to targeting specific Syrian military targets. The Pentagon on Thursday afternoon was presenting options through a series of exchanges with the White House, rather than through formal meetings.

US radar showed Syrian aircraft in the area at the time of the chemical attack. Tuesday's strike was believed to have been launched from Syria's Al-Shayrat air base in Homs, a senior regional security official told BuzzFeed News.

A network of local monitors who track airstrikes in Syria reached the same conclusion. According to an internal report by the group — which tracks fighter jets from takeoff to attack around the country in order to provide civilians with advance warning of airstrikes — the Syrian jets left Shayrat just before 6:30 a.m. and were then seen circling Khan Sheikhun before the attack. The name of the group is being withheld to protect the safety of its monitors.

Shayrat is a joint Russian-Syrian base that may include members of the elite Iranian Revolutionary Guard Corps, according to Jennifer Cafarella, a Syria analyst for the Washington-based Institute for the Study of War.

At present, the US military is debating whether to conduct such strikes with warships nearby or aircraft and drones in the air. BuzzFeed News witnessed several members of the Joint Chiefs — including Chief Naval Officer Adm. John Richardson and Chief of Staff of the Army Mark Milley — gathering in Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford's office on Thursday afternoon.

The US goal, it appears, is to signal to the regime that using sarin gas on civilians, as it is suspected of doing in Idlib on Tuesday, will not be tolerated, but stop short of military action that could lead to further escalation.

Jim Watson / AFP / Getty Images

US President Donald Trump speaks to the press on Air Force One

But any kind of intervention presents significant challenges and the Pentagon struggled to explain how it will get around them. There is no clear legal authority for the US military to strike the Syrian’s government’s air force, short of declaring war, and only Congress can declare that. Russia is unlikely to allow the passage of any UN Security resolution that would allow strikes on its ally, Syrian President Bashar al-Assad.

There are more than 1,000 US troops on the ground in Syria, working alongside local forces to train them for future operations against ISIS. A US strike on Syrian government forces could endanger those troops.

And Russian and Iranian forces are side-by-side with the Syrian military in parts the country, making a major campaign to take out the Syrian air force, without potentially striking Russia, all but impossible.

On Wednesday, Tillerson spoke with Russian Foreign Minister Sergei Lavrov about the chemical attack, a senior State Department official told reporters on Thursday. “He heard the Russian analysis,” of the attack, the official added without providing details. Russian officials have suggested that Syrian rebels are responsible for the attacks, something that Tillerson ruled out in his remarks on Thursday.

"There is no doubt that Syria, the Syria regime under the leadership of Bashar Al Assad are responsible for this attack and I think further it's very important that the Russian government consider carefully their continued support for the Assad regime,” he said.

Tillerson is headed to Moscow on Wednesday to discuss a range of issues with Lavrov such as Ukraine, North Korea and counterterrorism in the Middle East. But Syria is expected to loom large as the White House weighs a potential military strike on the Assad regime.

"If Russia chooses to continue to shield Assad on this issue I’m sure that will have a big impact on our thinking," the State Department official told reporters.

Though the US is leading a coalition against ISIS in both Iraq and Syria, a European official told BuzzFeed News that the US has provided “no indication yet on where or when” a strike against Assad may occur.

Secretary of Defense Jim Mattis traveled to Mar-a-Lago Thursday and will be there for the duration of Chinese President Xi Jinping's visit. Army Lt. Gen. H.R. McMaster, the national security adviser, traveled there with Trump as well.

In Washington, Sen. John McCain urged a military response to ground the Syrian air force: "It means that if the airplane flies, they get shot down. And if we need to, we could crater the runways, which would be easy enough to do. They're only flying out of six fields."

Should the US military decide to take out parts of the Syrian air capability — like the air strip or the planes that are believed to be part of Tuesday’s attack — that would only have a limited effect. The Syrian regime, for example, could simply repave the air strip.

McCain also said he believes Trump has the legal authority to strike Syria. "[Assad] dropped nerve gas on hundreds of innocent men, women, and children," McCain said, adding that Trump does "have a right to, he has an obligation to" intervene.

But there's little agreement on that point on Capitol Hill. Sen. Tim Kaine, a longtime advocate for a new Authorization for the Use of Military Force (AUMF) against ISIS, said Trump would have to go to Congress before any military strike. "There is no current legal authorization for military action against Syria," Kaine told BuzzFeed News. "If there's any contemplation about military action in Syria, the president would need to do what President Obama did and come to Congress with it."

Obama in 2013 ramped up for a military response to Syria's chemical weapons usage, only to back down at the last minute, when it became clear that Congress would not provide its support. Trump at the time tweeted repeatedly that Obama should not intervene, including in one tweet imploring the "very foolish leader" to not launch a military strike.

Sen. Ben Cardin, the ranking member on the Foreign Relations committee, said the administration could be in violation of the War Powers Act if it doesn't seek authority from Congress to intervene in Syria.

"He has no authority to go into Syria against the Assad regime," he said, saying that the president needs Congressional approval, preferably from a new AUMF tailored towards Syria’s chemical weapons. Congress, which is leaving for a two-week break after today, could stay in town to approve, Cardin said, if Trump were to submit such a proposal.

Sen. Bob Corker, the committee's chair, said he supports some sort of response to Syria. "I know they're developing those," he said. But on whether Trump has the authority legal authority for a military strike in Syria, he was more sanguine. "Typically presidents have the ability to make short-term steps," he said. "We're looking at that right now [...] I'm sure there's no way the president anticipates some longer effort, but let's wait and see what they propose."

Assad has repeatedly used chemical weapons against his own citizens since the civil war there erupted in 2011, most often deploying chlorine. But on Tuesday, he is believed to have used sarin nerve agent, based on photos and videos that emerged from Idlib that day.

In 2013, after the last time Assad was believed to have used sarin, Syria, in an agreement brokered by Russia following the US's unwillingness to strike the regime, agreed to destroy its chemical stockpiles under the watch of the international community. In November, Organization for the Prohibition of Chemical Weapons expressed grave concern that all gaps, inconsistencies and discrepancies” in the regime’s claims that it had fully destroyed it stockpiles.

Regardless, there is no clear stockpile site for the US to strike, leading to plans to strike the air force that carries out the attacks instead.

Nancy Youssef is a national security correspondent with BuzzFeed News and is based in Washington, DC.